Throw a comma in between figure and after, and you have a great logline. Sounds like a great Cinemax late night film.

EDIT:?After a sexual encounter, a teenage girl realizes she is being followed by a ghastly shapeshifter, and must retain a distance from it to stay alive, or sleep with someone new to pass on the deadly curse.

This movie really deserves a sequel.

Instead of 'relentlessly followed' (which is correct), I would say 'is being followed'. This more clearly suggests the moment she realises.

Perhaps?instead of "unknown supernatural figure", say something like "supernatural humanoid threat".
I would also prefer "she must keep a distance to stay alive, or have sex with someone to transfer the deadly curse." This indicates her dilemma.

This is a difficult one, because I believe she doesn't want to transfer the curse.
So her main goal is to stay alive, and keep a distance from the Being. This is an open goal, which doesn't always work.

I adore this movie.